1. Field of the Invention
The invention relates generally to the reduction of nitrogen oxides from flue gas, and, more particularly, to a combined SNCR/SCR process for reducing the level of nitrogen oxides from the flue gas of a fossil fuel fired furnace.
2. History of the Related Art
The combustion of fossil fuels (e.g. coal) in power plants to produce power generates undesirable nitrogen oxides (NOx), usually in the form of a combination of nitric oxide (NO) and nitrogen dioxide (NO.sub.2). It is known that under certain operating conditions the NOx level in a flue gas stream can be lowered by reacting the NOx with ammonia to produce harmless water and nitrogen as products. The NOx reducing reaction with ammonia can occur at relatively high temperatures, in the absence of a catalyst, in a process known as selective non-catalytic reduction (SNCR). The reaction can also occur at significantly lower temperatures, in the presence of certain catalysts, in a process known as selective catalytic reduction (SCR).
Several processes have been disclosed in prior art which attempt to combine an SNCR process with an SCR process. Typically, in the known so-called combined or "Staged" SNCR/SCR processes, a nitrogenous treatment agent, such as urea, is introduced within the boiler, at a location where the flue gas temperature is still high enough to effectively promote the non-catalytic reduction of NOx by ammonia, typically in the range of 1600.degree. F. to 2100.degree. F. To extract additional NOx reduction, catalysts are typically installed downstream from the location of the SNCR temperature region, at a point where the flue gas is at a temperature effective for the SCR process, typically in the range of 550.degree. F. to 780.degree. F. As the flue gas containing the excess ammonia remaining from the SNCR stage passes the catalyst, the excess ammonia reacts with the NOx. In most rationale approaches, as well as in the instant invention, reliance on simply the vagaries of the excess ammonia from "slip" passed from SNCR stage, is believed to be inefficient, non-reliable, and potentially harmful (i.e. uncontrolled ammonia slip and/or maldistribution). Further, by not relying solely on ammonia slip, or overloading, from the SNCR phase, the necessity for enhancers may be eliminated or greatly alleviated. Finally, limited tests on known SNCR/SCR systems which have been produced to date have illustrated that because of such factors as mal-distribution of NOx and ammonia at the face of catalyst from SNCR ammonia slip, the SCR performance is better enhanced by use of an independent ammonia injection grid to complement the ammonia supplied by slip from the SNCR stage (i.e. see the paper titled "Selective Catalytic Reduction Performance Project at Public Services Electric and Gas Company's Mercer Generating Station Unit No. 2", which was believed to have been presented at the Spring 1995 EPRI NOx Conference in Kansas City, Missouri. With the invention herein, and in other instances, the primary or supplemental supply of ammonia for the SCR phase, is from a selectively controllable source, independent of slip.
U.S. Pat. No. 4,302,431 to Atsukawa et al. discloses a process and apparatus for controlling nitrogen oxides in exhaust gases involving introducing ammonia into an exhaust gas at 700.degree. C. (1292.degree.) to 1300.degree. C. (2372.degree. F.), and then passing the exhaust gas over a catalyst at a temperature between 300.degree. C. (572.degree. F.) to 500.degree. C. (932.degree. F.) (preferably with the introduction of additional ammonia) to decompose remaining NOx and ammonia.
U.S. Pat. No. 4,978,514 to Hofmann et al. discloses a combined catalytic/non-catalytic process for nitrogen oxides reduction. The process of Hofmann et al. requires that, after the SNCR stage, sufficient ammonia is present in the effluent to react with the remaining effluent NOx in the catalyst stage. The Hofmann et al. process also utilizes an enhancer such as oxygenated hydrocarbons, heterocyclic hydrocarbons having at least one cyclic oxygen, sugar or molasses.
U.S. Pat. No. 4,981,660 to Leach discloses a selective hybrid NOx reduction process which utilizes an upright housing such as a natural draft heater tower. In the Leach process, a sufficient amount of reagent ammonia or ammonia radical must be added for the non-catalyst stage such that excess unreacted reagent remains for the catalyst stage.
U.S. Pat. No. 5,057,293 to Epperly et al. discloses a multi-stage process for reducing the concentration of pollutants in an effluent. The Epperly et al. multi-stage process reduces the nitrogen oxides concentration in the effluent such that an approximately 1:1 ratio of ammonia to nitrogen oxides remains in the effluent exiting the SNCR stage to provide ammonia for the SCR stage.
A related process is disclosed in U.S. Pat. No. 5,139,754 to Luftglass et al. The disclosed catalytic/non-catalytic combination process for nitrogen oxides reduction requires the introduction of a nitrogenous treatment agent for the non-catalytic stage in such an amount that ammonia remains in the treated non-catalytic effluent to be used for the SCR stage.
U.S. Pat. No. 5,233,934 to Krigmont et al. discloses a control method of reducing NOx in flue gas streams utilizing an SNCR treatment followed by an SCR treatment. The Krigmont et al. method tries to maximize the NOx removal in the SNCR stage, subject to certain ammonia slip restrictions, and injecting additional ammonia for the SCR stage
Another related patent, U.S. Pat. No. 5,286,467 to Sun et al. discloses a hybrid process for nitrogen oxides reduction in which a nitrogenous treatment agent other than ammonia is introduced in such a quantity that ammonia is present in the treated effluent leaving the non-catalytic stage. If the SNCR stage does not generate sufficient ammonia for the SCR portion, the process of the Sun et al. patent also provides a source of ammonia to make up the difference. U.S. Pat. No. 5,853,683 illustrates a process similar to the '467 patent.
U.S. Pat. No. 5,510,092 to Mansour et al. discloses a combined SNCR/SCR process in which SCR is employed for primary NOx reduction and NH.sub.3 is injected into the SNCR zone only when the NOx content of the SCR effluent exceeds a preselected design maximum value.
The known hybrid SNCR/SCR related processes typically attempt to maximize the efficiency and chemical utilization of the SNCR so as to minimize the level of nitrogen oxides remaining in the flue gas for processing by the SCR stage, while simultaneously producing excess NH.sub.3. A known deficiency of such an SNCR/SCR processes is that the intentional injection of excess SNCR reagent is not the least cost methodology of providing for ammonia reagent within flue gas.
A review of the above described prior art clearly illustrates that, in instances where mandated levels of post combustion NOx reduction do not require full scale stand alone SCR systems (i.e. see U.S. Pat. No. 5,853,683), combined SNCR/SCR systems of the general type discussed above may be a reasonable solution. Furthermore, to better insure reliability, uniformity, flexibility, responsiveness and overall system control, the most appropriate combined SNCR/SCR system, will include a source of ammonia for the SCR stage of the system, which is at least in part, separate from any dependence on ammonia slip (natural or forced) from the SNCR stage.
While combined SNCR/SCR systems hereinbefore have to some extent addressed the need to provide a separate ammonia supply for the SCR stage, this very requirement of providing an independent ammonia supply has greatly inhibited this approach. In this regard, the SNCR stage is typically performed by injecting urea solution into the furnace where relatively high temperatures serve to initiate the breakdown of urea to form the transient species, including ammonia, which lead to effective NOx reduction. As such, in a typical urea based SNCR system, there is no need to transport, handle and store ammonia on the plant site. On the other hand it has been suggested, from time to time, to attempt to utilize ammonia, rather than urea, as the basic feedstock in an SNCR system however, because of the rapid reaction of ammonia at the high temperatures within a boiler, effective distribution across the boiler cross section may only be performed with high flow of carrying media (i.e. air or steam), and has not proved to be economical in many situations. Thus, inasmuch as direct feed of urea to SCR is not a reasonable substitute for the direct feed of ammonia to the SCR injection grid, nor is the direct feed of ammonia to a boiler for SNCR a practical approach, herein lies the problem of past approaches of having to handle both benign urea and toxic ammonia if an SCR assist to an SNCR system was desired.
Ammonia for SCR uses such as described above, is generally delivered to power plants in the form of anhydrous ammonia, or aqueous ammonia. Ammonia is gas at ambient temperatures and pressures, and is normally shipped and stored as a liquid, either in pressure vessels at ambient temperature, and high pressure (i.e. over 16 bars ), or in refrigerated vessels at ambient or nearly ambient pressure, and at about -33.degree. C. It is transported in bulk in ships, barges, and railroad tank cars, and in tank trucks on public roads and highways. It is frequently stored in large quantities at industrial sites in populated areas and is frequently used as the working fluid in large refrigeration systems. It is now coming into wider use for the removal of NOx from flue gas at power generating stations in urban areas.
Anhydrous ammonia is an extremely hazardous, toxic, and volatile material. In the event of an accidental discharge, it can cause immediate death to humans and animals and rapid death to trees and plants. Both anhydrous liquid ammonia, and concentrated aqueous liquid ammonia, display a deadly characteristic which substantially increases the risk of widespread injury and death in case of a spill. Specifically, upon sudden release to the atmosphere, as might occur in a sudden and accidental discharge (i.e. a storage failure at a power plant, a train wreck, or a traffic accident), and the pressure and temperature are sufficiently high, the liquid ammonia will become airborne as a mixture of very fine liquid droplets that do not fall to the ground, provided that no obstacles are encountered in the immediate vicinity of the release. The droplets evaporate quickly as air is entrained. The evaporation process cools the air so that a cold mixture of air and ammonia vapor is formed (i.e. a dense cloud of poisonous ammonia in a potentially deadly concentration). Unlike gaseous ammonia, which, though toxic, is lighter than air and quickly dissipates to harmless concentrations, the cloud can persist for a surprisingly long time, as long as several hours, before it finally disappears. The cloud is typically heavier than air and tends to drift along the surface of the earth, i.e., the ground or the surface of a body of water. The cloud moves with the wind and can sweep over a total area, i.e., a "footprint," much larger than the area covered by the cloud at any one moment. Contact with the cloud can be instantly incapacitating, and a single breath may be fatal. Substantial numbers of bulk shipments of anhydrous ammonia routinely move through or near densely populated areas. It is roughly estimated that an anhydrous ammonia spill from a 40,000 pound truck trailer would generate a cloud having an average lethal footprint of 29 acres, that is, an area of 29 acres in which the concentration of ammonia would reach a lethal level, about 0.5 percent, before the cloud eventually dissipated.
In addition to the inherent danger of storing, transporting and handling large quantities of ammonia, the expense insofar as safety aspects, insurance costs, specialized training, and the difficult to quantify emotional exposure of living and working next to a such potential catastrophe, it is apparent that if another, less hazardous commodity could be transported instead of ammonia, and then be readily used instead of transported aqueous or anhydrous ammonia, the hazards and/or expenses associated with ammonia shipment and handling would be considerably reduced. To some extent, attempts have been made in the supply of ammonia for NOx control in power plant environments by substituting concentrated aqueous liquid ammonia for anhydrous ammonia. Such a solution has achieved only limited success, due to any number of factors, for example: the high energy cost of vaporizing the water carrier, relatively costly storage facilities; and, even though aqueous ammonia is safer to handle than anhydrous ammonia, it is still very difficult and costly to handle in a safe manner.
Urea is an ideal candidate as an ammonia substitute. Urea is a non-toxic chemical compound and, for purposes of this discussion, presents essentially no danger to the environment, animals, plant life and human beings. It is solid under ambient temperatures and pressures. Consequently, urea can be safely and inexpensively shipped in bulk and stored until it is used to produce a gaseous mixture containing ammonia. It will not leak, explode, be a source of toxic fumes, require pressurization, increase insurance premiums, require extensive safety programs, or be a concern to the plant, community and individuals who may be aware of the transportation and/or storage dangers of ammonia. Best of all, in the invention herein, which incorporates an improved SNCR/SCR combined system, urea is already present for the SNCR phase.
By utilizing the urea feedstock and the teachings of the SNCR/SCR systems to date, and by further incorporating the teachings of the present invention of integrating a urea to ammonia sequence in the process, the hereinabove discussed deficiencies of prior systems are overcome or, in the least, greatly alleviated. Thus the invention herein teaches a concept where the advantages of an improved combination SNCR/SCR system can be achieved without the necessity of transporting and storing aqueous or anhydrous ammonia.